The present invention relates to particle accelerators of the type known as tandem accelerators.
Tandem accelerators are devices for producing beams of positively charged particles with high energies of the order of 10 MeV. The basic design of such devices is very simple. A negative ion beam, for example H.sup.- or O.sup.- is accelerated to a central positively biased electrode at which point a stripper foil removes two or more electrons from the negatively charged ions to produce a positively charged ion beam which is subsequently accelerated to the ground plane by the same potential as that used to produce the positive ions. In order to achieve ion energies of about 10 MeV it is necessary to apply potentials of about 5 MeV to the central electrode and so it is necessary to isolate the central electrode from ground in such a manner as to be capable of withstanding the potential gradients produced. To achieve this in air requires the separation of the high and low potential planes by distances of many metres.
In an attempt to reduce the size of tandem accelerators, designers of tandem accelerators therefore conventionally fill the accelerators with a gas such as sulphur hexafluoride, which has considerably better breakdown characteristics than those of air. However, even so, the distance required to hold-off even 1 MeV is about 2 metres at one bar. Also, sulphur hexafluoride is potentially dangerous and very expensive, both of which characteristics lead to considerable engineering complexity in the design of tandem accelerators. Furthermore, the ion beam has to be isolated from the rest of the apparatus by means of cells and windows which are thin enough to be penetrated by the ion beams but strong enough to withstand the pressure gradient between the near vacuum inside the cells, required to maintain the ion beams, and the pressurised environment necessary to provide the electrical isolation of the positively charged central eletrode. Both these windows and the stripper foils are susceptible to damage, which requires taking the accelerator out of service to rectify, which both is inconvenient and costly as the equipment has to be virtually dismantled for this to be done.